A Thin Layer can be applied to either exterior or interior boundaries of the physics interface. However, there are some noticeable differences between adding a thin layer on an exterior or on an interior boundary.

On an exterior boundary, the bottom of the thin layer is attached to the adjacent domain. This means that the thickness of the thin layer extends outward from the domain in the direction of the normal. For a nonlayered thin structure, an exterior displacement field is introduced to create the slit, so it is possible to apply loads and boundary conditions to this exterior field. For example, applying a fixed constraint on an exterior thin layer makes it equivalent to a Spring Foundation. For a layered thin structure, the top side of the layered material is always the exterior. Loads and boundary conditions can be applied to the top side only.